The present invention relates to a method for making oxydiphthalic anhydride involving contact between nitrophthalic anhydride and a catalytic amount of a dialkylaminopyridine in the presence of a nonpolar organic solvent under reflux conditions.
Oxydiphthalic anhydride, and moreparticularly 4,4'-oxydiphthalic anhydride, is a well-known monomer useful in the synthesis of polyimides having good thermal properties and high solvent resistance. Oxydiphthalic anhydride or "ODAN" has been prepared by heating 4-nitrophthalic anhydride with a sodium or potassium nitrite in a dipolar aprotic solvent, as shown by Japanese Patent Document No. 80/136,246 (Chem. Abstracts 95:42680 (1981). Another procedure for making ODAN is shown by Schwartz et al., U.S. Pat. No. 4,697,023, incorporated herein by reference, employing a halophthalic anhydride, water, and an alkali metal compound such as potassium fluoride or potassium carbonate in the presence of a dipolar aprotic solvent. An additional procedure for making ODAN is based on the hydrolysis of the corresponding 4,4'-oxybis(N-methylphthalimide) which can be made by the procedure of Markezich et al. "Reactions of Fluoride and Nitride Ions With 4-Nitrophthalimide", Journal of Organic Chemistry 42, 3431 (1977), or as shown in copending application Ser. No. 881,415, filed July 2, 1986, now U.S. Pat. No. 4,780,544.
Although the above-described procedures can be used to make ODAN, employment of a dipolar aprotic solvent, such as dimethylsulfoxide, is economically unattractive while the hydrolysis procedure from the corresponding bisimide requires an additional processing step.
A further procedure for making certain aromatic ethers is shown by Brunelle et al., U.S. Pat. No. 4,513,141, by effecting the displacement of reactive radicals on an activated aromatic nucleus using a dialkylamino branched alkyl-substituted pyridinium salt as a phase transfer catalyst. Although improved yields of bis(aromatic ethers) are achieved, a phase transfer catalyst must be used which is not commercially available, and which requires several steps to make.
The present invention is based on the discovery that ODAN, and preferably 4,4'-oxydiphthalic anhydride, can be made by heating 4-nitrophthalic or 4-fluorophthalic anhydride in the presence of dialkylaminopyridine and a nonpolar solvent such as dichlorobenzene. Unexpectedly, the reaction can be done in a nonpolar solvent, in the absence of an added phase transfer catalyst.